1. Field of the Invention
The present invention relates to a permanent magnet type (hybrid type) stepping motor which is suitably used in such office automation machines and apparatuses as printers, high-speed facsimile machines and copying machines of plain paper copier (PPC) type.
2. Description of the Prior Art
Of prior art permanent type stepping motors, two-phase stepping motors have been predominantly used as taught, for example, in U.S. Pat. No. 4,675,564.
FIGS. 1A and 1B shows a structure of a two-phase, permanent type stepping motor shown in the above U.S. Patent.
In the drawing, the stepping motor includes a stator housing 1, a stator iron core 2, and a stator winding 3. The iron core 2 comprises magnetic poles 2-1 to 2-8 each having pole teeth 2-10 provided on its inner periphery. The stator winding 3 comprises windings 3-1 to 3-8 wound around the corresponding magnetic poles. The iron core 2 and the winding 3 make up a stator S.
The illustrated motor also includes end brackets 4 and 4, bearings 5 and 5, a rotor shaft 6, rotor magnetic poles 7 and 8, and a permanent magnet 9. The magnetic poles 7 and 8 are provided on its outer peripheries with pole teeth 7-10 and 8-10 respectively. These constituent members 6 to 9 make up a rotor R.
The two-phase, permanent magnet type stepping motor, however, has had problems 1 to 4 as follows.
1 The maximum number of lead wires is 4, but this motor requires at least 8 transistors in a driver circuit. PA1 2 The motor produces high torque ripples and thus much vibrations. PA1 3 There are low cost motors using four poles, while motors for accurate use require at least eight poles. PA1 4 In order to obtain a very small angle, many pole teeth as many as 100 or more must be provided on the rotor magnetic poles, which makes it difficult to form such pole teeth on the magnetic poles. PA1 1 The maximum number of lead wires is 5, but its driver circuit requires at least 10 transistors. PA1 2 The necessary number of magnetic poles is at least 10. Thus, this 5-phase motor becomes more costly than the 2-phase motor. PA1 3 For the purpose of obtaining a very small angle, a stator must be constructed so as to be symmetrical with respect to a point. However, the stator of point symmetrical type brings about another problems, that is, corrective operation being widely used for offsetting blanking errors cannot be employed. In more detail, in an ordinary type motor, core laminates are stacked being shifted at a certain angle, e.g. 90 degrees, each time the core laminates have been stacked to have a predetermined thickness, so as to prevent the core blanking errors from being accumulated, however, the point symmetrical arrangement mentioned above does not permit such angular shifting during the stacking of core laminates. For this reason, it is impossible to accommodate the core blanking error. PA1 4 Since the motor requires at least 10 magnetic poles, it is difficult to increase greatly the number of turns in a winding and thus it is impossible to obtain a sufficient torque. PA1 (1) The magnetic poles of the stator are set at 6 in number and arranged equally with an identical pitch. PA1 (2) The pitch T.sub.s of the pole teeth provided at the tip ends of the respective magnetic poles of the stator is set to be equal to the pitch T.sub.R of the pole teeth of the rotor. PA1 (3) An angle .theta..sub.r made between one of the pole teeth of the stator magnetic poles and one of the pole teeth of rotor magnetic poles opposed thereto is set to satisfy a relationship .theta..sub.r =120.degree. /Z, said pole tooth number Z of the rotor magnetic poles is set to satisfy a relationship Z=6n.+-.4 (where n is an positive integer). PA1 (4) The motor is driven with 3, 6, 7 or 9 lead wires. PA1 (5) The pitch .tau..sub.s of the pole teeth of the stator magnetic poles and the high pitch .tau..sub.R of the pole teeth of the rotor should satisfy the following correlations. EQU .tau..sub.s =180.tau..sub.R /(180.+-..tau..sub.R) EQU 60/.tau..sub.s =m (where m=1, 2, . . .)
For the purpose of eliminating the above problems, a 5-phase, permanent magnet type stepping motor has been proposed. However, this stepping motor has had the following problems 1 to 4. That is: